Health: Hooked on the exercise high

Claire Smith didn't see anything wrong with her exercise routine. The daily three-hour runs seemed normal, as did the subsequent hours spent in the pool, on her bike, training for the triathlon or at the gym.

"When it opened at 6.40am, I was there," she says. "And I definitely wouldn't go out at night because, it would affect my early morning training." To most people, her daily routing would have sounded like horrendously hard work. "That's the thing," she says. "To me it didn't seem like that much at all."

Such is society's acceptance of sport that, even knowing Smith was addicted to exercise, it's still hard not to feel jealous of her enthusiasm for a good work-out. Yet such an addiction can be damaging. On the physical side, too much exertion can leave our bodies vulnerable to injury and unable to fight disease and infection. Socially, it can lead to broken marriages, lost jobs and neglected friends and family.

With hindsight, Smith realises this. "I didn't have a relationship for a long time because of my obsession with exercise, and my career was affected because I only took jobs that would allow me to do the routine I wanted," she says. "I'd train if I was ill or injured but I never thought there was something wrong. I just thought I really loved doing sport."

The medical world is not short on theories explaining why the drive to exercise can become so compulsive that it is actually harmful. Some point to the chemical properties of a strenuous work-out, including an endorphine-induced "high": a number of reports have likened the effect to that of the anti-depressant drug Prozac. Others show that exercise can lower anxiety, improve mood and provide a sense of mastery and self-esteem.

None of which sounds particularly damaging in the short term: after all, sport is something that complements and enhances our lives. The difference for the exercise addict is that sport is their life.

Susan Van Scoyoc, a counselling psychologist, treats exercise addicts, but says most go unnoticed. "It is seen as a kind of healthy addiction which, of course, is a contradiction in terms. It can be as damaging and all-consuming as a drug or alcohol addiction, but most people don't recognise it because of its association with healthy living."

Van Scoyoc relates a story of a young girl who, even while preparing food or taking a bath, felt she had to keep exercising and moving. "She would even hide in her bedroom doing press-ups," says Scoyoc. "Her behaviour was affecting the whole family."

As with drug and alcohol dependencies, a sporting obsession can provide a sort of quick-fix high that feeds a habit. Smith says she enjoyed "the really good, fit body that people would comment on", and describes how, mid-routine, she "felt really good about myself" - a feeling that waned as soon as she paused to stop and think. For this reason, some psychologists see exercise addiction as having as many psychological triggers as physical ones and believe that, as a habit, it is just another way of manifesting a problem of low self-esteem.

"We treat exercise addicts no differently from any other addicts," says Andrew Vincent, the addictions treatment programme manager at the Priory hospital in Sturt, Surrey. "They would be in group therapy with alcoholics, drug addicts, shopping addicts and all kinds of other addicts, because it boils down to the same thing - a way of avoiding emotional pain and events."

Even if they aren't laughed out of the therapy room by the more "hardcore" addicts - "before they get to know each other that's almost always the initial reaction," says Vincent - exercise junkies face further obstacles. Where others can set their sights on total abstinence, therapists and patients realise that this impossible for exercise-dependent people who can easily slip back into their old routine.

"We encourage all our patients to get out and about during the day, but we did have to voice concerns about one woman who was doing early morning running with the rest of the group. It was OK for them, but she needed to be a bit careful - just like an alcoholic drinking with friends. You can't just start up again because everyone else is doing it."

The common thread linking many exercise addicts is that they are female. "It almost always goes hand-in-hand with anorexia and a poor body image," says Van Scoyoc.

Twelve years ago, when Jessica Davies's boyfriend made a snide comment about her weight, her confidence hit an all-time low and she became stuck in a cycle of excessive daily exercise in an attempt to lose weight. "I was 22 and I went down to about seven stone. When my mum saw me she was horrified and took me to the doctor straight away," she says. "I've got over my addiction now but I think I will always be conscious I need to exercise to keep my weight to a stable level."

Smith says she was vulnerable because of what she now recognises as "a classical addicted personality". Just like other addictions she has suffered, she remembers how her habit crept up. "I just needed to do more and more and more. Eventually it all came to a halt because I had children and I had no option: it had to stop. But otherwise I don't know how long I might have gone on like that."

Like Davies, she believes her habit was a mask for a much deeper emotional issue. "I was literally running away from my problems, but exercise just became another one of those. I realise now it wasn't a solution at all."